1. Field of the Invention
The present invention relates to detection circuits and methods for use in telephone systems and, more particularly, to a composite ringing and coin control voltage detection circuit and method for loop testing, wherein the circuit will detect either an AC ringing voltage or a DC coin control voltage without the need for software control or bulky circuitry.
2. Description of the Prior Art
In telephone systems, mechanized loop testing (MLT) is utilized to verify connections between a central office terminal (COT) and a remote terminal (RT) and, additionally, to test circuit functionality at the RT. A test access unit (TAU), which is located at the RT, must be capable of detecting four different ringing voltages (+R, -R, +T and -T) and two distinct coin control voltages (+CC and -CC). .+-.R may be detected by the TAU on the `ring` line, while .+-.T and .+-.CC may be detected on the `tip` line at the RT. For each unique signaling state detected by the TAU, an appropriate termination is applied between the tip, ring and ground of the RT channel unit. The termination applied at the RT is sensed at the COT to verify the system's functionality.
As can be seen in Table 1, one of four terminations must be applied at the RT for each of the six different signals which are detected by the TAU. An absorptive termination is applied at the RT when +R is detected on the ring line, while a reflective termination is applied when -R is detected on the ring line. An absorptive positive tip party identification (PTPI) termination is applied at the RT if either +T or +CC is detected on the tip line, and a reflective negative tip party identification (NTPI) is applied if either -T or -CC is detected on the tip line. Thus, the detection circuit at the TAU must be able to distinguish between +R and -R on the ring line and +T (or +CC) and -T (or -CC) on the tip line and output an appropriate logic signal to allow the corresponding termination to be applied.
TABLE 1 ______________________________________ Signal Nominal Signal Line Termination Voltage DC Offset ______________________________________ +R Ring Absorptive 100 VRMS +48 V -R Ring Reflective 100 VRMS -48 V +T Tip Absorptive PTPI 100 VRMS +48 V -T Tip Reflective NTPI 100 VRMS -48 V +CC Tip Absorptive PTPI +130 VDC N/A -CC Tip Reflective NTPI -130 VDC N/A ______________________________________
As shown in Table 1, each signal appears on either the ring or the tip lines. The ringing signals, +R, -R, +T, and -T are typically 100 VRMS at 20 Hz. +R and +T are offset with approximately +48 VDC, and, conversely, -R and -T are offset with -48 VDC. +CC and -CC are .+-.130 VDC signals respectively.
The four different signal types (positive and negative offset ringing voltages and positive and negative coin control voltages) to be detected at the TAU are illustrated in the graph in FIG. 1. The signals are displayed in peak-to-peak voltage (in volts) on the vertical, or y, axis 10, with respect to time (in ms) on the horizontal, or x, axis 12. Two sinusoidal lines 14 and 16 represent the positive and negative offset ringing voltages, .+-.R and .+-.T, which appear on the ring and the tip lines respectively at the TAU. The two horizontal lines 18 and 20 represent the positive and negative coin control voltages, .+-.CC. Also shown in dashed lines on the graph are maximum values 22 and 26 and minimum values 24 and 28 for .+-.R and .+-.T, and maximum values 30 and 36 and minimum values 32 and 34 for .+-.CC, which are to be detected at the remote terminal.
As can be seen in FIG. 1, the ringing signal, which is applied to the ring and tip lines, is nominally a 20 Hz, 100 VRMS signal. This AC signal is superimposed on either the positive battery voltage +48 VDC, which is shown as signal 14 (+R or +T), or the negative voltage -48 VDC, or signal 16 (-R or -T). This offset is attributable to the +48 VDC battery source in a telephone system. The AC ringing signals 14 and 16 have a maximum signal level of 120 VRMS, as indicated by dashed lines 22 and 26, respectively, and a minimum signal level of 70 VRMS, as indicated by dashed lines 24 and 28, respectively.
The coin control voltage signals, on the other hand, are DC signals with values of .+-.130 V as represented by horizontal lines 18 and 20 (+CC and -CC respectively). Dashed lines 30, 32, 34 and 36 represent the maximum and minimum values (.+-.200 VDC and .+-.70 VDC) for .+-.CC.
Each of these signals must be detected within its threshold so that the appropriate termination may be applied. Most existing systems require separate detection circuits for detecting the coin control voltages and the ringing voltages, as in Brolin et al, U.S. Pat. No. 4,277,647, which teaches the use of two distinct circuits: a coin control voltage detector and a ringing detector. The prior art systems utilize either bulky discrete analog circuitry or digital circuitry using a microprocessor with elaborate software as the detection means. Additional microprocessor support with software control may be required in some cases. Although those circuits which utilize a microprocessor may save some board space and components, a heavy software programming effort is required, and processor time is wasted.
The amount of detection circuitry utilized in the existing systems would be reduced and simplified if a single, or composite, circuit could be used to detect both the ringing and coin control voltages. However, a problem arises in such composite detection circuits, in that false detection flags may be set. This can most clearly be seen in FIG. 2.
A negative ringing signal 16 (-T) along with minimum and maximum signals 28 and 26 (-Tmin and -Tmax) are shown in FIG. 2. As can clearly be seen, -T 16 may be detected when it crosses a -T detection threshold 38, which is set at the most negative point of -Tmin 28. In a composite circuit, i.e., a circuit which will detect both .+-.T and .+-.CC, -T 16 would also trip any threshold which would be used to detect both +CC or -CC because such thresholds must be set, at a minimum, at .+-.CCmin 32, 34 or .+-.70 VDC. In such a circuit, both the reflective NTPI termination flag would be set (for -T and -CC) and the absorptive PTPI termination flag would be set (for +CC). Because only the reflective NTPI termination should be set, as shown in Table 1, the absorptive PTPI termination flag would be falsely triggered.
Because of the false detection problem, existing systems utilize either bulky circuitry to discriminate between the two types of signals, or digital circuitry using a microprocessor with complex software for measuring, for example, rise and fall times of the detected signal to discriminate between the AC and DC signals. Naturally, writing and debugging software consumes time, while running the software consumes processor time. Consequently, it would be desirable to utilize a simple composite detection circuit for detecting both ringing and coin control signals without false detection or the need for bulky circuitry or elaborate software.